The "Fallen Hero" thing only works, IMHO, if the hero in question is no longer being used. The fact that the X-series was ongoing throws quite a monkey wrench into that, which is one point Inti actually brought up in the "X changed to Copy X" discussion. It makes any chronologically previous character development, which is still being worked on, absolutely meaningless if you already know that it's all going to end in a 180 turn during the series-gap.
Copy X MK1 is himself a character that I grew rather fond of, because he's the kind of villain I love to hate. He's not just evil, his powers of suggestion are among the most dangerous we've seen. Most serious villains believe that they're doing the right thing, but to turn previous heroes (the Big Four) to your cause, not through coersion or infection, but through name recognition and rationalization? That's when you know you're dealing with somebody sick; his methods to corrupt others is something unique to what we usually see in the MegaMan story.
It also really does fly in the face of X's purpose, the whole theme of viewing human and robotic life equally. It's something they made a point of emphasizing in later X-series games, in Inti's own Zero-series concepts (the infamous Three Keys, for example), and which also eases the inevitable tie-in to Legends, where we find artificial beings virtually indistinguishable from the real thing. The effects of making X the villain would cause ripples well beyond the Zero-series alone. Good or bad, it's something to consider.
Heck, if you really want to, you don't even have to make X that much of a bad guy. You can make him even more tragic, by making him a character who created that horrible policy of Repliroid disposal, because he simply had no other choice, and was forced into it by the bad situation Neo Arcadia and humanity in general was in. In other words, make him do something he clearly hates. He's fighting for it, even though he knows it's wrong.
They basically did that with Harpuia, making him the most "X-like" of the Big Four across Z2 and Z3.
Also, I stand by what I said earlier; review the entire ZERO-series, see how much of it relies on Cyber Elf X. The entire foundation of the series' greatest moments comes crumbling down without Zero's undying devotion toward X's cause.
So be it... But that's why we are the best partners... I'll do what you want... Rest for a while. I will handle it, you can count on me. I won't stop! When an enemy appears... I'll terminate it..
I have to agree. X himself is a tremendous part of Zero's motivation, reiterated throughout the X-series all the way up until Zero4. Their bond is a pretty heavy foundation for both characters, across both series. Had X truly flipped, then combined with Zero's amnesia, it would cause Zero's own character to warp. His future character developments would be difficult if not impossible to tie back to the Zero we're familiar with from XXX years ago.
In itself, this wouldn't necessarily be bad, but it does pose quite a risk of alienation when Inti has already screwed with Zero's design to the furthest degree that Inafune would let them get away with, and wiped his memory on top of that, meaning the changes to his personality would tie in no way whatsoever to his past experiences. And that's the problem. There's a difference between character growth and character reboot. Zero's look and memory are screwed to hell; it's his motivations that keep his identity intact throughout the Zero series. Otherwise we're left with "Mega Man Red Guy With A Sword".
Also, on great moments:
I appreciate Copy-X existence since it prevented that real X got killed by Zero.
"...the original X was mightier than you!"
I can't tell you how much I love that line. Taking someone who got on top of the world by abusing a true hero's name, and putting him in his place.